Orthogonal frequency division multiplexing

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DISADVANTAGES OF RADIO PROPAGATION
1. path loss
2. fading
3. Doppler shift
4. multipath delay spread
 Orthogonal frequency-division multiplexing (OFDM) is a frequency-division multiplexing (FDM) scheme utilized as a digital multi-carrier modulation method. A large number of closely-spaced orthogonal sub-carriers are used to carry data. The data is divided into several parallel data streams or channels, one for each sub-carrier. Each sub-carrier is modulated with a conventional modulation scheme (such as quadrature amplitude modulation or phase shift keying) at a low symbol rate, maintaining total data rates similar to conventional single-carrier modulation schemes in the same bandwidth.



HISTORY OF OFDM

The origins of OFDM development started in the late 1950’s  with the introduction of Frequency Division Multiplexing (FDM) for data communications. In 1966 Chang patented the structure of OFDM  and published the concept of using orthogonal overlapping multi-tone signals for data communications. In 1971 Weinstein  introduced the idea of using a Discrete Fourier Transform (DFT) for implementation of the generation and reception of OFDM signals, eliminating the requirement for banks of analog subcarrier oscillators presents an opportunity for an easy implementation of OFDM, especially with the use of Fast Fourier Transforms (FFT), which are an efficient implementation of the DFT. Until the late 1980’s that work began on the development of OFDM for commercial use, with the introduction of the Digital Audio Broadcasting (DAB) system.

CHARACTERISTIS AND PRINCIPLE OF OPERATION
ORTHOGONALITY

In OFDM, the sub-carrier frequencies are chosen so that the sub-carriers are orthogonal to each other, meaning that cross-talk between the sub-channels is eliminated and inter-carrier guard bands are not required. This greatly simplifies the design of both the transmitter and the receiver; unlike conventional FDM, a separate filter for each sub-channel is not required. The orthogonality requires that the sub-carrier spacing is Δf = k/(TU) Hertz, where TU seconds is the useful symbol duration (the receiver side window size), and k is a positive integer, typically equal to 1.
Therefore, with N sub-carriers, the total passband bandwidth will be B ≈ N·Δf (Hz).The orthogonality also allows high spectral efficiency, with a total symbol rate near the Nyquist rate.

IMPLEMENTATION USING THE FFT ALGORITHM

The orthogonality allows for efficient modulator and demodulator implementation using the FFT algorithm on the receiver side, and inverse FFT on the sender side. Although the principles and some of the benefits have been known since the 1960s, OFDM is popular for wideband communications today by way of low-cost digital signal processing components that can efficiently calculate the FFT.
GUARD INTERVAL FOR ELIMINATING ISI
One key principle of OFDM is that since low symbol rate modulation schemes (i.e. where the symbols are relatively long compared to the channel time characteristics) suffer less from intersymbol interference caused by multipath propagation, it is advantageous to transmit a number of low-rate streams in parallel instead of a single high-rate stream. Since the duration of each symbol is long, it is feasible to insert a guard interval between the OFDM symbols, thus eliminating the intersymbol interference. The guard interval also eliminates the need for a pulse-shaping filter, and it reduces the sensitivity to time synchronization problems.The cyclic prefix, which is transmitted during the guard interval, consists of the end of the OFDM symbol copied into the guard interval, and the guard interval is transmitted followed by the OFDM symbol. The reason that the guard interval consists of a copy of the end of the OFDM symbol is so that the receiver will integrate over an integer number of sinusoid cycles for each of the multipaths when it performs OFDM demodulation with the FFT.
Cyclic prefix (CP): eliminate ISI and ICI
CP transforms a linear convolution channel to a cyclic convolution channel Orthogonality over dispersive channel if CP is long enough.CP introduces a loss in SNR to mitigate interference.

SIMPLIFIED EQUALISATION

Some of the sub-carriers in some of the OFDM symbols may carry pilot signals for measurement of the channel conditions, i.e. the equalizer gain and phase shift for each sub-carrier. Pilot signals and training symbols may also be used for time synchronization (to avoid inter-symbol interference, ISI) and frequency synchronization (to avoid inter-carrier interference, ICI, caused by Doppler shift).

CHANNEL CODING AND INTERLEAVING

OFDM is invariably used in conjunction with channel coding (forward error correction), and almost always uses frequency and/or time interleaving. The reason why interleaving is used on OFDM is to attempt to spread the errors out in the bit-stream that is presented to the error correction decoder, because when such decoders are presented with a high concentration of errors the decoder is unable to correct all the bit errors, and a burst of uncorrected errors occurs. The information is typically FEC encoded and interleaved prior to modulation The bits carried by faded subcarriers might be detected in error without the FEC With FEC and interleaving, erroneous bits may be correctable, thereby providing frequency diversity. A common type of error correction coding used with OFDM-based systems is convolution coding.

ADAPTIVE TRANSMISSION

The term discrete multitone modulation (DMT) denotes OFDM based communication systems that adapt the transmission to the channel conditions individually for each sub-carrier, by means of so called bit-loading. Examples are ADSL and VDSL. The upstream and downstream speeds can be varied by allocating either more or fewer carriers for each purpose. Some forms of Rate-adaptive DSL use this feature in real time, so that the bit rate is adapted to the co-channel interference and bandwidth is allocated to whichever subscriber that needs it most.

OFDM EXTENDED WITH MULTIPLE ACCESS

OFDM can be combined with multiple access using time, frequency or coding separation of the users.In Orthogonal Frequency Division Multiple Access (OFDMA), frequency-division multiple access is achieved by assigning different OFDM sub-channels to different users. OFDMA supports differentiated quality-of-service by assigning different number of sub-carriers to different users in a similar fashion as in CDMA, and thus complex packet scheduling or media access control schemes can be avoided. OFDMA is also a candidate access method for the IEEE 802.22 Wireless Regional Area Networks (WRAN).

LINEAR TRANSMITTER POWER AMPLIFIER

An OFDM signal exhibits a high peak-to-average power ratio (PAPR) because the independent phases of the sub-carriers mean that they will often combine constructively. Handling this high PAPR requires:
A high-resolution digital-to-analog converter (DAC) in the transmitter
A high-resolution analog-to-digital converter (ADC) in the receiver
A linear signal chain.
Any non-linearity in the signal chain will cause intermodulation distortion that raises the noise floor  may cause inter-carrier interference  generates out-of-band spurious radiation.

TRANSMITTER
 RECIEVER
Versions of OFDM

MIMO OFDM
Multiple Input, Multiple Output Orthogonal Frequency Division Multiplexing is a technology developed by Iospan Wireless that uses multiple antennas to transmit and receive radio signal.According to Iospan,
"In this environment, radio signals bounce off buildings, trees and other objects as they travel between the two antennas. This bouncing effect produces multiple "echoes" or "images" of the signal. As a result, the original signal and the individual echoes each arrive at the receiver antenna at slightly different times causing the echoes to interfere with one another thus degrading signal quality.
The MIMO system uses multiple antennas to simultaneously transmit data, in small pieces to the receiver, which can process the data flows and put them back together.

VOFDM (VECTOR OFDM)
VOFDM (Vector OFDM) uses the concept of MIMO technology and is also being developed by Cisco Systems.

WOFDM (WIDEBAND OFDM)
WOFDM - Wideband OFDM, developed by Wi-Lan, develops spacing between channels large enough so that any frequency errors between transmitter and receiver have no effect on performance.

FLASH-OFDM
Flash-OFDM (Fast Low-latency Access with Seamless Handoff Orthogonal Frequency Division Multiplexing), which is also referred to as F-OFDM, is a system that is based on OFDM and specifies also higher protocol layers. It has been developed and is marketed by Flarion. Flash-OFDM has generated interest as a packet-switched cellular bearer, where it would compete with GSM and 3G networks.

Applications

WIRED APPLICATION
ADSL and VDSL broadband access via POTS copper wiring.
Power line communication (PLC).
Multimedia over Coax Alliance (MoCA) home networking.
ITU-T G.hn, a standard which provides high-speed local area networking over existing home wiring (power lines, phone lines and coaxial cables).
The wireless LAN radio interfaces IEEE 802.11a, g, n
The digital radio systems DAB/EUREKA 147,
The terrestrial digital TV system DVB-T.
The terrestrial mobile TV systems DVB-H,
The cellular communication systems Flash-OFDM
The Wireless MAN / Fixed broadband wireless access (BWA) standard IEEE 802.16 (or WiMAX).

Advantages 
Can easily adapt to severe channel conditions without complex equalization
Robust against narrow-band co-channel interference
Robust against Intersymbol interference (ISI) and fading caused by multipath propagation
High spectral efficiency
Efficient implementation using FFT
Low sensitivity to time synchronization errors
Tuned sub-channel receiver filters are not required (unlike conventional FDM)

Disadvantages
Sensitive to Doppler shift.
Sensitive to frequency synchronization problems.
High peak-to-average-power ratio (PAPR), requiring linear transmitter circuitry, which suffers from poor power efficiency.
Loss of efficiency caused by Cyclic prefix/Guard interval

COMPLEXITY COMPARISON of TDMA, CDMA & OFDM
  PERFORMANCE ANALYSIS OF TDMA,CDMA & OFDM
CONCLUSION
Performs better than a single modulated carrier in multipath fading
With a properly implemented guard interval:
     – Time waveform appears periodic
    – orthogonality of subcarriers is ensured
    – ISI and ICI are eliminated

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